1. Field of the Invention
The present invention relates to a slip control apparatus of a vehicle and, more particularly, to a slip control apparatus of a vehicle, as an automobile, adapted to control and prevent a slip of driven wheels of the vehicle and to improve a running stability.
2. Description of Related Art
Japanese Patent Application (laid-open) No. 31,864/1988 proposes a slip control apparatus of such a type that a slip of the driven wheels is effectively controlled and prevented by detecting a slip value of the driven wheels versus the undriven wheels and by carrying out a feedback control of a driving torque acting on the driven wheels at a given response velocity so as to change the slip value of the driven wheels to a target slip value, thereby preventing an action of an excessive driving torque. The slip control apparatus of this type is designed such that, when a friction coefficient.mu. on a road surface is low as on a road covered with snow, a response velocity of the feedback control is set low to lessen a change of driving torque and to become unlikely to cause a slip of the driven wheels. On the contrary, this apparatus is such that, when a road surface is more slippery, a response velocity in the direction of increasing the driving torque acting on the driven wheels is made smaller, thereby becoming unlikely to cause slipping.
U.S. Pat. Nos. 4,779,696 and 4,762,196 disclose another type of slip control apparatus. The slip control apparatus of the type disclosed in U.S. Pat. No. 4,779,696 is such that an actual slip value is subjected to feedback control to amount to a given target value and that the target value and an initial value of the target value are altered in accordance with a running state. The slip control apparatus of the type disclosed in U.S. Pat. No. 4,762,196 is such that a slip control is carried out by reducing output from the engine and applying a braking force to the driven wheels when an actual slip value of the driven wheels is large while it is carried out merely by reducing output of the engine when the actual slip value is small.
For the slip control apparatus as disclosed in Japanese Patent Publication No. 31,864/1988, if the driven wheels slip again to a relatively large extent during running on a road having a low friction coefficient .mu., the feedback control of the driving torque is carried out at a low response velocity corresponding to the low friction coefficient .mu. so that the driving torque acting upon the wheels is gradually adjusted to be decreased by means of the feed-back control. Accordingly, it will take a longer period of time in order to converge this slip that has occurred again so that demands have been made to control a slip as rapidly as possible from the very early stage of the slip of the driven wheels, thereby improving performance of converging the slip.